1. Field of the Invention
The present invention relates to an image reduction apparatus which converts a pixel density of image data to obtain reduction image data of an original image.
2. Related Background Art
A facsimile apparatus as a typical conventional still image communication apparatus employs a method of sequentially scanning, encoding, and transmitting an image. In this method, since all the data of an image must be transmitted to recognize an entire image, a long transmission time is required, and it is difficult to apply this method to image communication services such as an image database service, a videotex, and the like, in which an image must be quickly judged. In order to realize these services, unlike in the method employed in the facsimile apparatus, hierarchical encoding methods such as a sequential reproduction encoding method ("Sequential Reproduction Encoding Method of Facsimile Signal Suitable for Conversational Image Communications", Endo, Yamasaki, et al., Shingakuron (B), J67-B. 12, pp. 1462-1469 (1984)) are proposed. In the sequential reproduction encoding method, rough image information is transmitted, and thereafter, additional information is transmitted to generate detailed image data. We also proposed similar encoding methods in U.S. Ser. Nos. 398,665, (filed on Aug. 25, 1989), 400,103 (filed on Aug. 29, 1989), 401,240 (filed on Aug. 31, 1989), 438,082 (filed on Nov. 20, 1989), 446,791 (filed on Dec. 6, 1989), and 458,362 (filed on Dec. 28, 1989).
FIG. 1A shows a conventional hierarchical encoding system.
The system shown in FIG. 1A includes frame memories 701 to 704 for respectively storing a 1/1 image, and 1/2, 1/4, and 1/8 reduction images, reduction units 705 to 707 for generating 1/2, 1/4, and 1/8 reduction images, respectively, and encoders 708 to 711 for encoding 1/8, 1/4, 1/2, and 1/1 images, respectively.
The reduction unit 705 reduces an image from the frame memory 701 in both main and sub scanning directions by subsampling, or the like to generate a 1/2 image, and stores the 1/2 image in the frame memory 702. Furthermore, the reduction unit 706 reduces the 1/2 image to form a 1/4 image, and stores the 1/4 image in the frame memory 703. Similarly, the reduction unit 707 generates a 1/8 low-resolution image, and stores it in the frame memory 704.
Since images are sequentially encoded and transmitted from those having a lower resolution, a rough entire image can be quickly recognized. In the system shown in FIG. 1A, an image is reduced to 1/2, 1/4, or 1/8 in both the main and sub scanning directions. Encoding is performed in the order of 1/8, 1/4, 1/2, and 1/1 (original image), and encoded images are transmitted in this order. In order to encode a 1/8 image, a 1/8 image stored in the frame memory 704 is sequentially scanned, and the encoder 708 performs entropy coding, e.g., arithmetic coding with reference to a pixel of interest and surrounding pixels. For a 1/4 image, the encoder 709 performs encoding with reference to surrounding pixels of a pixel of interest from the frame memory 704 and surrounding pixels of the 1/8 image from the frame memory 704, thereby improving coding efficiency. Similarly, a 1/2 image in the frame memory 702 is encoded by the encoder 710 with reference to a 1/4 image in the frame memory 703, and an original image in the frame memory 701 is encoded by the encoder 711 with reference to a 1/2 image in the frame memory 702.
In an apparatus other than still image communication apparatuses, reduction of binary images is executed. For example, a binary image is reduced when an image is output from a single image database to printers having different output resolutions. When a binary image which is read at 400 dpi is output to a 300- or 200-dpi printer, an image is reduced to 3/4 or 1/2 in both the vertical and horizontal directions.
Conventionally, when such reduction is executed, subsampling for thinning pixels at predetermined intervals, or a method of re-binarizing pixels after low-pass filtering to subsample pixels is employed.
In the hierarchical encoding method, since reduction images are sequentially encoded and transmitted in the order starting from an image having a lower resolution, as described above, an entire image can be transmitted early. Therefore, information of a reduced low-resolution image must be left to facilitate recognition of an entire image.
When such reduction is executed by a conventional method, information may be lost. FIG. 1B shows an example wherein pixels indicated by marks "x" of an original image (1) are subsampled to obtain an image (2) which is reduced to 1/2 in both the vertical and horizontal directions.
When only subsampling is executed, if a single line L is present between sampling points (marks "x"), this line is lost by reduction. In order to solve such a drawback, a method of performing subsampling after filtering is proposed. FIG. 1C shows this example. In FIG. 1C, marks "x" indicate sampling points. In FIG. 1C, 3.times.3 low-pass filtering having coefficients (3) in FIG. 1C is executed, and a filtered output is binarized. For example, if a filtered output represents 8 or more, it can be defined as binary "1"; otherwise, it can be defined as binary "0". However, in the method of using filtering, when a vertical line L2 in an original image shown in FIG. 1C is present between subsampling lines, it is also lost.
Therefore, in a system which repeats reduction several times, a line having a one-pixel width is finally lost in a low-resolution image unless it is saved. Thus, a thin line such as a one-pixel line must be saved. The similar requirement is posed for a dither processing image.
Since thin lines, especially, characters or the like, are ordinarily constituted by black thin lines, a saved state of information of a reduction image varies between a case wherein an input image is directly reduced and a case wherein an input image is subjected to negative/positive reversal processing once, and then is reduced.
Such an example will be described below using an arrangement for reducing an image size to 1/2 by a projection method. A low-pass filter used in the projection method has weighting coefficients shown in FIG. 56. The center of this filter is caused to coincide with a mark "o" in FIG. 57 to perform filtering, and a filtering result is binarized using 8/16 as a threshold value, thereby generating a reduction image having a 1/2 image size.
Assume that line segments are detected by a 3.times.3 pixel block of a low-pass filter. FIGS. 58A and 58B show examples of line segments. FIG. 58A shows a portion like a crossing portion of line segments of a character "K". When this image is binarized based on the filtering result of the low-pass filter, the binarization result indicates "black", resulting in a preferred result.
However, an original image shown in FIG. 58B is, e.g., a part of an arc, and when this image is binarized based on the filtering result of the low-pass filter, the binarization result undesirably indicates "white". Thus, this result is corrected to indicate "black".
In this manner, the images shown in FIGS. 58A and 58B are negative/positive reversal processing results. Thus, the same method can be applied to one image but cannot be applied to the other image upon generation of a reduction image. More specifically, when a negative image is subjected to the same processing for a positive image, if a character image in an original image is a negative image such as a microfilm, information of a character converted to a white thin line is lost when thin lines are lost.